1. Field of the Invention
The present invention relates to a method for manufacturing a liquid crystal display (LCD) device, and in particular, the present invention relates to a method for an LCD in which the pad terminal communicating an electrical signal to an outer device and the terminal of the outer device cohere well with each other, and to the structure of an LCD having the same pad terminal.
2. Description of Related Art
The cathode ray tube (CRT), the most widely used display device, is being replaced by the thin flat display device because the flat display device is thinner and lighter than the CRT so it can be applied to any place. Active research activities have focused on the development of liquid crystal display devices because of their high resolution and fast response time suitable for displaying motion picture images. Furthermore, the active panel comprising an active switching element such as a thin film transistor (or TFT) is more popularly applied to the LCD.
A liquid crystal display device works by using polarization and optical anisotropy of a liquid crystal. By controlling the orientation of liquid crystal molecules having rod shape through polarization technique, transmission and interception of a light through the liquid crystal are achieved due to the anisotropy of the liquid crystal. This principle is applied to the liquid crystal display device. Active matrix LCDs (AMLCDs) having TFTs arranged in a matrix pattern and pixel electrodes connected to the TFTs provide high quality images and are now widely used.
The structure of a conventional AMLCD will now be described. FIG. 1 shows a perspective view of the AMLCD and FIG. 2 shows the cross-sectional view of FIG. 1 along the cutting line II—II. The conventional AMLCD comprises an upper panel 3 and a lower panel 5 which are joined to each other with a liquid crystal material 10 injected therebetween. The upper panel 3 has a color filter panel which includes a sequential arrangement of red(R), green(G) and blue(B) color filters 7 on a first transparent substrate 1a at pixel positions designed in a matrix pattern. Among these color filters 7, black matrixes 9 are formed in a lattice pattern. The black matrixes 9 prevent the colors from mixing at the boundary area. On the color filters 7, a common electrode 8 is formed. The common electrode 8 is one electrode of the two electrodes generating an electric field applied to the liquid crystal layer.
The lower panel 5 of the LCD comprises switching elements and bus lines generating the electric field for driving the liquid crystal layer. This panel is called an active panel. The active panel 5 of an AMLCD includes pixel electrodes 41 designed in a matrix pattern and formed on a second transparent substrate 1b. Along the column direction of the pixel electrodes 41, signal bus lines 13 are formed, and along the row direction of the pixel electrodes 41, data bus lines 23 are formed. At a corner of a pixel electrode 41, a TFT 19 for driving the pixel electrode 41 is formed. A gate electrode 11 of the TET 19 is connected with the signal bus line 13 (or the gate line). A source electrode 21 of the TET 19 is connected with the data line 23 (or the source line). A semiconductor layer 33 is formed between the source electrode 21 and the drain electrode 31. An ohmic contact exists between the source electrode 21 and the semiconductor layer 33 and between the drain electrode 31 and the semiconductor layer 33 are also ohmic contacted. A gate pad 15 and a source pad 67, the terminals of the bus lines, are formed at the end portion of the gate line 13 and the source line 23, respectively. Additionally, a gate pad terminal 57 and a source pad terminal 25 are formed on the gate pad 15 and the source pad 67, respectively.
As the signal voltage applied to the gate pad 15 is applied to the gate electrode 11 via the gate line 13, the TFT 19 of the corresponding gate electrode 11 transitions to the ON state. Then the source electrode 21 and the drain electrode 31 of the TFT 19 are electrically connected so that the electrical picture data applied to the source pad 25 is sent to the drain electrode 31 through the source line 23 and the source electrode 21. Therefore, by controlling the signal voltage to the gate electrode 11, the transfer of picture data to the drain electrode is controlled. That is, the TFT 19 acts as a switching element. A gate insulating layer 17 is inserted between the layer including the gate electrode 11 and the layer including the source electrode 23 to electrically isolate them. A passivation layer 37 is formed on the layer including the source line 23 to protect all elements of the transistor.
The color filter panel 3 and the active panel 5 are bonded together to face each other with a certain separation distance therebetween (i.e., a cell gap). Liquid crystal material 10 fills the cell gap and the edge of the bonded panels is sealed with a sealant 81 such as an epoxy to prevent the liquid crystal from leaking out so that a liquid crystal panel of an AMLCD is completed.
The AMLCD is finally made by assembling the liquid crystal panel with peripheral devices for the screen data. At this time, the pads of the liquid crystal panel and the terminal of the peripheral devices are generally electrically connected with a tape carrier package (TCP) using an anisotropic conductive film (ACF). FIG. 3 shows a general structure of the ACF. FIGS. 4a and 4b illustrate the conventional method for connecting the TCP to the pad using the ACF and illustrate the structure of the pad.
As shown in FIG. 3, the ACF 71 comprises a plurality of conductive ball 95 coated with an insulation membrane 93 in an isotropic film 31. On the pad terminals 47 connected to the pads 45 (for example, the gate pads 15 or the source pad 67) at the edge of the liquid crystal panel, an ACF 71 is attached and TCP 73 is sequentially attached thereon. At this time, the conductive pad 75 of the TCP 73 should be aligned with the pad 45 (for example, the gate pads 15 or the source pad 67) of the liquid crystal panel, as shown in FIG. 4a. The TCP 73 is pressed and heated while the conductive balls 95 are inserted between the TCP pad 75 and the pad terminal 47 of the liquid crystal panel. When sufficient pressure is applied against the TCP 73, the insulation membrane 93 covering the conductive ball 95 are broken so that each TPC pad 75 becomes electrically connected to each pad terminal 47 of the liquid crystal panel, as shown in FIG. 4b. Even if there are some conductive balls 95 between the neighbored pad terminals 47, the neighbored pad terminals 47 are electrically isolated from each other because the conductive balls 95 are covered by the insulation membrane 93.
In the step of attaching the TCP to the pad terminal as mentioned above, the film portion 77 between each pad portion 73 are expanded somewhat by heat and pressure and cohered to the passivation layer 37 formed on the top of the liquid crystal panel. As shown in FIG. 5, after removing the pressure and the heat, the expanded film portion of the TCP is shrunk which results in the pulling force 83 so that the passivation layer 37 being cohered with the film portion 77 is peeled off.
Generally, after the liquid crystal panel is completed, the edge portion of the panel having the shorting bar used for protecting the electrostatic need to be trimmed off. At that time, the trimming force, which is applied to the trimmed edge, can cause the passivation layer 37 or the gate insulating layer 17 to be structurally unstable. At this portion, the passivation layer 37 can be easily peeled off, when the heating energy is removed after the film portion 77 of the TCP is cohered with the passivation layer 37 with the ACF 71 therebetween. This comes from the peeling force 89 made of the vector summation of the horizontal shrinking force 87 of the ACF 71 and the vertical shrinking force 85 of the ACF 71 and TCP 73, as shown in FIG. 6.